The present invention is directed to the measurement of flow velocity, in general, and more specifically to a LIDAR system and method of measuring flow velocity in three axes.
A common flight hazard of any aircraft operating near the Earth is the potential for collision with ground structures and obstacles. Helicopters, in particular, and now new classes of aircraft known as unmanned air vehicles (UAVs), often operate less than five hundred feet above ground level (AGL). In this environment, it is not uncommon for these aircraft to collide with electrical power lines, support wires for radio towers, or various structures and obstacles. These collisions typically result in loss of life, significant aircraft damage, damage to the structures or obstacles themselves, subsequent loss of power distribution on the electrical grid, and danger to persons and property on the ground. Aircraft, such as helicopters and UAVs, for example, typically operate in these low altitudes for takeoff and landing, various low-level military maneuvers, and commercial applications, such as electrical utility inspection or emergency rescue missions.
Inspecting electrical power lines from an aircraft requires flying close to the Earth along high tension power lines and support structures looking for damaged equipment. Use of helicopters permit electric utility inspection crews to cover a large area of the power grids over a short period of time. Other helicopter applications which require low flying flight profiles include emergency and rescue missions, medical emergencies, border surveillance, and supply of floating oil platforms, for example. Likewise, UAV applications require autonomous control for surveillance, take-off, landing and delivery of munitions. In all of these applications, the flight crew and aircraft are at risk of colliding with obstacles like power lines, cables, towers, and other similar support structures. The risk becomes even greater with poor visibility and flights over unknown terrain. Depending on the type of aircraft canopy, the lighting, and the environmental conditions, many obstacles may become effectively invisible to the pilot and crew due to background clutter even under daylight conditions. Also, because of the narrow field of view offered the pilot by the aircraft, some obstacles may not be seen until it is too late for avoidance. Surprisingly, the highest accident rates are typically associated with clear conditions which indicates that during reduced states of pilot situational awareness, identification of hazardous ground obstacles may occur less regularly.
Some helicopters are equipped with structural wire strike protection kits which are fitted on the front end of the aircraft and intended to force a wire in the path of the aircraft to slide over the top or under the bottom of the aircraft. However, for this device to be effective, a contacted wire must slide across the canopy and into the wire cutters. When this occurs, the wire is likely to be severed by the wire cutter(provided it meets certain size and strength envelopes), freeing the aircraft from the hazards. It is not uncommon for electrical utility companies to identify cut wires but have no report of a wire strike accident. In some cases this indicates the flight crew did not know they hit a wire, much less cut it, or are reluctant to report the incident. However, if the wire does not slide across the canopy, and impacts other areas of the helicopter such as the rotors or landing skids, the wire cannot be severed by the wire strike protection system. As tension builds in the wire due to the forward motion, damage to the aircraft ensues with penetration into the canopy and flight crew, damage to the main rotor resulting in an imbalance, or loss of tail rotor control. In all these cases, the flight crew is in immediate life threatening danger. Depending upon the degree of interaction, fatalities can be attributed to the high-g accelerations of the rotor imbalance, blunt force trauma due to subsequent impact with the ground/aircraft, or harmful interactions with the wire resulting in significant lacerations or electrocution. Accordingly, due to the many low-level flying applications and the increasing risks posed thereby, obstacle avoidance warning systems for these aircraft have become of paramount importance for the safety of the pilot and crew of the aircraft. These devices are intended to warn the flight crew in advance of the collision with the obstacle, so that they(or an automated flight control system)can take evasive action prior to collision.
Amphitech International of Montreal, Canada, has developed a radar based obstacle awareness system named OASYS which was presented at the Quebec HeliExpo 2001. While it is proposed that OASYS can detect small obstacles, such as power lines, for example, up to two kilometers away even in adverse weather conditions, it is a rather heavy, bulky and costly unit, which may render it prohibitive for small aircraft usage.
Another obstacle awareness warning system is being developed by Dornier GmbH, in its Defense and Civil Systems Business Unit of Friedrichshafen, Germany under the tradename of HELLAS (Helicopter Laser Radar). In this unit, a laser beam is sequentially scanned through a line series of approximately one hundred optic fibers to create a raster line scan which is projected from the system. The line scan is steered vertically by a pivoted, oscillating mirror. The field-of-view is approximately plus and minus 32 degrees in azimuth and elevation with respect to a line of sight of the system. While Dornier promotes HELLAS as being an effective obstacle detection unit, it remains a relatively narrow field of view device that is rather complex and costly. In addition, the large number of optic fibers required for effective obstacle detection resolution, appears to render the device difficult to repeatedly align which may lead to manufacturing difficulties.
Another problem encountered in these low-level flight profile aircraft applications is the wind or air flow conditions surrounding the aircraft while it is carrying out its tasks. In some cases, an aircraft may encounter substantially different air-flow conditions from side to side. For example, when flying in a canyon, the aircraft may have a mountain wall on one side and open spaces on the other. Landing on the flight deck of an aircraft carrier poses similar risks. Such uneven air flow conditions may have an adverse affect on the responsiveness of the aircraft to the avoidance of detected obstacles.
Accordingly, it is desireable to have a wide field scanning laser based obstacle awareness system which is simpler in design and less costly than its predecessors to render it an economically attractive safety system for low-level flight profile aircraft. Combining air flow and obstacle measurements in a common system would provide the knowledge of air conditions surrounding the aircraft when an obstacle is detected in its flight path allowing a pilot to make his avoidance decisions based on such air data information. An enhanced situational awareness display would augment the peripheral vision of the flight crew to potential collision obstacles. The present invention is intended to provide for these desirable features in a laser based obstacle awareness system as will become more evident from the description thereof found herein below.
In accordance with one aspect of the present invention, a light detection and ranging (LIDAR) system for measuring flow velocity in three axes comprises: a LIDAR arrangement of optical elements for generating a coherent beam of light and directing said coherent beam of light substantially on a first optical path incident on at least one rotationally operated optical element which directs said coherent beam of light from said system along a second optical path of a predetermined pattern, the at least one rotationally operated optical element also for receiving reflections of the coherent beam of light from particles along the predetermined pattern and directing the beam reflections to the LIDAR arrangement of optical elements; the LIDAR arrangement of optical elements for directing the beam reflections to a light detector which converts the beam reflections into representative electrical signals; and processing means coupled to the light detector for detecting electrical signal bursts from the electrical signals, each signal burst being representative of light beam reflections from at least one particle substantially at a corresponding position along the predetermined pattern, and for computing a Doppler frequency for each of a selected plurality of detected electrical signal bursts from the signal content thereof, the processing means for associating the selected plurality of detected electrical signal bursts with their corresponding positions along the predetermined pattern and for computing a three axis flow velocity measurement from at least three of the selected plurality of computed Doppler frequencies and their corresponding positions along the predetermined pattern.
In accordance with another aspect of the present invention, a method of measuring flow velocity in three axes comprises the steps of: generating a coherent beam of light and directing the coherent beam of light into space substantially along an optical path of a predetermined pattern; receiving reflections of the coherent beam of light from particles in space along the predetermined pattern; converting the light beam reflections into representative electrical signals; detecting electrical signal bursts from the electrical signals, each signal burst being representative of light beam reflections from at least one particle substantially at a corresponding position along the predetermined pattern; computing a Doppler frequency for each of a selected plurality of detected electrical signal bursts from the signal content thereof, associating the selected plurality of detected electrical signal bursts with their corresponding positions along the predetermined path; and computing a three axis flow velocity measurement from at least three of the selected plurality of computed Doppler frequencies and their corresponding positions along the predetermined path.